A thermoelectric element is a device capable of transforming a temperature difference into an electric voltage and vice versa. At least two thermoelectric elements may be thermally and electrically coupled to one another to form a thermoelectric module. A thermoelectric module may include two types of thermoelectric elements, for example a thermoelectric element composed of a P-type material and another thermoelectric element composed of an N-type material. The P-type material may have a Seebeck coefficient that is more positive than the Seebeck coefficient for the N-type material. The N-type material may have a Seebeck coefficient that is more negative than the Seebeck coefficient for the P-type material.
A thermoelectric module powered by an external electrical power source may operate as a heat pump, with thermal energy absorbed at one set of thermoelectric junctions and released at another set of thermoelectric junctions. Alternatively, a thermoelectric module receiving thermal energy from an external heat source at one set of thermoelectric junctions while having a lower temperature heat sink coupled to another set of thermoelectric junctions may generate electrical power available for coupling to an external electrical load.
A thermoelectric element is preferably formed from materials that efficiently transform an applied temperature differential into electrical energy, or conversely, electrical energy into a temperature differential, depending on the application and arrangement of elements in a thermoelectric module. Thermoelectric properties of a thermoelectric element may be described by any one or more of the Seebeck coefficient, thermal conductivity, and electrical resistivity. Preferred properties of a thermoelectric element include chemical and mechanical stability under intended operating conditions, mechanical strength, for example compression strength, and the ability to form low-loss electrical and thermal connections with other thermoelectric elements and other parts of a thermoelectric module.
Previously known methods and equipment for transforming raw materials into commercially useful thermoelectric elements have been complex and expensive to build and operate. For example, in the zone melting method, a common previously known method for volume production, a protective coating of graphite is applied to an ampule made of quartz or similar glass. The ampule is then arranged vertically, precursor materials for the thermoelectric element are loaded into the ampule, and a seed crystal may be placed in the bottom of the ampule, where melting will begin when the ampule is heated by movable heating elements. The movable heating elements may be arranged in a ring capable of travelling along the length of the ampule, for example from the bottom towards the top of the ampule. As the heater moves along the ampule, material in the ampule melts in a zone near the heater. Melted material cools as it emerges from the melted region and crystallizes into a solid having a higher density than the mixture of constituent precursor materials loaded into the ampule. The crystallized material is then removed from the ampule as an ingot and remnant graphite removed from the ingot.
After removal from the ampule, the ingot is cut into slices and the slices cut into individual thermoelectric elements. The thermoelectric elements may need additional shaping or polishing, for example to remove saw marks, before the thermoelectric elements are assembled into a thermoelectric module. Some of the material from the ingot is lost during cutting and any subsequent shaping and polishing operations needed to produce thermoelectric elements having preferred finished dimensions. Other material from the ingot may be lost because pieces removed near the sides or ends of the ingot may not have a shape suitable for use in a thermoelectric module.
Another previously known processing method for forming thermoelectric elements includes subjecting a mixture of powders to compression at elevated temperatures to form an ingot from which the thermoelectric elements are cut. For example, in US Patent application 2010/0051081 by Iida et al., magnesium and silicon raw materials are melted in an alumina crucible for three hours at 1100° C. in a reducing atmosphere of 5% hydrogen, then pulverized to a powder having particles small enough to pass through a 30 micrometer mesh. The powder is compressed in a spark plasma sintering system under 17 to 30 Mpa of pressure at 800 C, holding for 10 minutes. The resulting ingot may then be sawed into individual thermoelectric elements. Electrically conductive plates may be joined to the solid object resulting from processing of the powder, before or after slices from the ingot are sectioned. The conductive plates provide a path for heat energy and electrical energy to flow through the thermoelectric element. The long, high temperature melting step, as well as the complex, serial nature of a spark plasma sintering press operation result in a complex material formation process.
Various methods have been applied to the formation of solid objects from components in powder form. For example, in metal injection molding, powdered metal, possibly mixed with a binder to form a feedstock material with desired flow properties, is injected into a mold at relatively high pressure. The molded part is removed from the mold and then heated to drive off any binders and cause bonding between the metal particles in the molded part. Shrinkage of about five percent may occur when comparing a finished part to mold dimensions.
Powdered metallurgy is a family of processes used to form a solid object from component powdered materials. Powdered materials are compressed to form an intermediate, high-porosity solid referred to as a green body or a green compact. The green compact is subjected to heat treatment to cause bonding between the particles in the mixture of powdered constituents, leading to a reduction in porosity and densification of the solid object. When heat treatment is conducted without bringing the material to melting or the point of liquefaction, this process may be referred to as sintering. The maximum temperature during heating of the green compact may be less than the melting point temperature of at least one of the constituents of the mixed powder.
The formation of a green compact or green body is common to several previously known methods for densifying powdered materials. During sintering, a green body may increase in size or decrease in size depending on the combination of constituent powders, processing temperatures, and other factors. Some sintering processes are known to cause shrinkage of a green body. For example, a green body including ceramic materials or metals such as titanium may be expected to shrink up to about 20 percent when subjected to some previously known sintering processes.
Other processes such as hot pressing and powder metallurgy may be used to form a solid object from powdered constituents. These processes apply compression forces to a powder heated to form a solid object. Compression may be achieved by compression forces acting from outside the powdered mixture, for example by tamping with a plunger or similar mechanical compression device or by subjecting the powder to high pressure in a closed vessel.